1. Field of the Invention
The invention pertains to lithographic processes for fabricating devices.
2. Art Background
Lithographic processes play an important role in the manufacture of devices such as semiconductor devices. During the manufacture of these devices, lithographic processes are used to pattern substrates such as silicon wafers or processed silicon wafers which are, for example, wholly or partially covered by metal, silicon dioxide, or polycrystalline silicon. For example, a substrate is coated with an energy-sensitive material called a resist. Selected portions of the resist are exposed to a form of energy which either removes the exposed portions to bare portions of the substrate or more typically induces a change in the solubility or reactivity of the exposed portions in relation to a given developing agent or etchant. The more soluble or reactive portions of the resist are removed and portions of the substrate are bared by applying the developing agent or etchant to the resist. The bared portions of the substrate are then treated, e.g., are etched, implanted, or metallized.
Organic polymer resists are presently the most commonly employed commercial resists. It is desirable in using these resists to achieve a high resolution pattern with good linewidth control (e.g., a pattern having linewidths smaller than about 1.5 .mu.m and linewidth variations smaller than about 10 percent). This requisite generally requires that the resist have a thickness which is both small (less than about 1 .mu.m) and substantially uniform (thickness variations should be less than about 10 percent). Resist layers having small, substantially uniform thicknesses are readily formed on substrates having planar surfaces, e.g., the surface of an unprocessed silicon wafer, using conventional spin-deposition techniques. But in the case of substrates having nonplanar surfaces, e.g., the stepped surface of a processed silicon wafer, the necessary thickness uniformity is generally achieved using relatively thick resist layers that preclude the formation of high resolution patterns.
The desire to achieve high resolution patterns with good linewidth control in substrates having nonplanar surfaces has led to the use of multi-level, e.g., trilevel, resist configurations. Typically, in these configurations, a layer of an organic polymer (which need not be energy-sensitive), e.g., a novalac resin, thick enough to yield a planar surface, is spin-deposited onto a nonplanar substrate surface. (Planar, in this context, means that a tangent plane to any point of the upper surface of the planarizing layer forms an angle with a least-squares-fit planar approximation to the substrate surface which is less than or equal to about 30 degrees.) A layer of silicon dioxide is deposited onto the planarizing layer using conventional rf sputtering or plasma-enhanced chemical vapor deposition (CVD) techniques. Then, a layer of energy-sensitive material, typically organic polymer resist, e.g., photoresist, e-beam resist, ion beam resist, or x-ray resist, thin enough to yield the desired resolution, is spin-deposited onto the silicon dioxide layer. A desired pattern (to be transferred into the substrate) is defined in the top resist layer by conventional exposure and development. This pattern is then dry etched, e.g., plasma etched or reactive ion etched, into the underlying silicon dioxide layer using a plasma which includes fluorine-containing etchant species, such as a plasma struck in an atmosphere containing CHF.sub.3 and O.sub.2 (or air), while employing the patterned top resist layer as an etch mask. The patterned silicon dioxide layer is in turn used as an etch mask during the reactive ion etching of the underlying planarizing layer, the etching occurring in a plasma struck in, for example, an O.sub.2 atmosphere. The patterned resist is finally employed to process the substrate by using the patterned planarizing layer as a mask, e.g., an etch, implantation, or metallization mask.
The trilevel resist has proven to be highly useful for patterning substrates having planar and/or nonplanar surfaces. However, a desire to reduce processing cost has precipitated a search for materials which are functionally equivalent to the silicon dioxide but which are deposited by relatively inexpensive techniques.
Materials which have been considered as alternatives to silicon dioxide include organosilicon glass resins which are deposited using inexpensive, conventional spin-deposition techniques. (An organosilicon glass resin, for present purposes, is a polymer, having a noncrystalline structure, which includes silicon, oxygen, carbon and hydrogen.) After baking, these spin-on glass resins have etch characteristics essentially equivalent to those of silicon dioxide, e.g., they are readily plasma or reactive ion etched in, for example, CHF.sub.3 and O.sub.2 (or air) plasmas.
Unfortunately, the organosilicon glass resins undergo lateral (transverse to the depth direction) etching during the etching, e.g., O.sub.2 reactive ion etching, of the planarizing layer. This lateral etching, which typically occurs at a rate equal to or greater than about 0.05 .mu.m/minute, results in not entirely advantageous linewidth control during etching of the planarizing layer. Variations in linewidth greater than about 10 percent (produced during the etching of the planarizing layer) are generally observed.
Materials containing tantalum or titanium atoms chemically bound to oxygen atoms have also been considered as substitutes for silicon dioxide. It is possible to deposit these materials by conventional spin-deposition techniques, and they undergo relatively little (as compared to the spin-on glass resins) lateral etching during the etching of the planarizing layer. But relatively thick (thicker than about 0.05 .mu.m) layers of some of these materials tend to crack during processing. Moreover, these materials often etch relatively slowly (compared to the organosilicon glass resins) in a plasma struck in, for example, CHF.sub.3 and O.sub.2 (or air). Consequently, the relatively thin, top resist layer (which functions as the etch mask for these materials) undergoes significant degradation during the etching of these materials, which also results in a loss of linewidth control during pattern transfer into the substrate. Relatively thin (thinner than about 0.05 .mu.m) layers of these materials (which are etched in less time than thick layers) have been used to avoid this problem. However, these layers have many pinholes (more than about 10/cm.sup.2) that produce unwanted features during pattern transfer. Thus, these materials are not now being actively investigated.
Alternatives to the silicon dioxide layer of the trilevel resist which are formed using relatively inexpensive techniques, which do not lead to a loss of linewidth control, and are substantially free of defects such as pinholes have not been found.